Renoir’s “Madame Léon Clapisson” (1883), with its current state on the left & digitized restoration on the right (all images courtesy Art Institute of Chicago)

Art as we see it now isn’t always as the artist intended. After the paint dries, there’s still chemistry happening on the canvas.

An exhibition currently at the Art Institute of Chicago is exploring one such alteration of a 19th century Renoir. Renoir’s True Colors: Science Solves a Mystery, which opened earlier this month, shows the research and digital restoration of “Madame Léon Clapisson” (1883), where a bug-based vivid red that once swam through the background of the portrait is now a more mellow mottled sea of greens and grays.

Richard Van Duyne of Northwestern University presented the research on the painting at the 2014 meeting of the American Association for the Advancement of Science (AAAS). The process of deciphering the original hues of the painting involved Surface Enhanced Raman Spectroscopy (SERS), which unlike the x-ray fluorescence often used in art conservation was able to use incredibly small particles of the paint to reveal their composition. As Van Duyne told Chemistry World, a magazine published by the Royal Society of Chemistry:

“The recolorized version of what we imagine the painting was like is based on molecular analysis. […] There are already many ways to analyse the inorganic content of paintings and the elemental composition, and now we add molecular resolution to the whole enterprise.”

Conservators and scientists use a high-power microscope and X-rays to examine the pigment particles and individual brushstrokes of Renoir’s “Madame Léon Clapisson”

So why did the red color fade so severely? The pigment was something called a “lake,” where an organic color is attached to an inorganic color. In this case, the red actually came from cochineal insects, scaly bugs often used in carmine colors. Unfortunately due to light exposure over time, the organic and inorganic separated. This was revealed for “Madame Léon Clapisson” when a small band of bright red was found along the frame of the painting where it had been protected by the light.

These kinds of pigments are known as “fugitive” due to their fleeting nature, and aren’t just an issue for Renoirs. The analysis is part of an ongoing partnership between Northwestern University and the Art Institute of Chicago, which last year was enhanced by a $2.5 million grant from the Andrew W. Mellon Foundation that set up the Northwestern University/Art Institute of Chicago Center for Scientific Studies in the Arts. Previous research in the partnership showed fugitive colors faded in Winslow Homer’s “For to Be a Farmer’s Boy” (1887).

Other contemporary painters of the time also have had similar fading in their work. Vincent van Gogh’s flowers in “Roses” (1890) at the National Gallery of Art were originally pink, but now have faded to white. However, van Gogh and likely other artists of his time weren’t unaware of this problem. As he wrote to his brother Theo:

“Paintings fade like flowers. […] All the colors that Impressionism has brought into fashion are unstable, so there is all the more reason to simply use them too brightly — time will tone them down only too much.”

All sorts of factors could impact the survival of the original colors, from the sturdiness of the paper to the conditions in which it was stored. Being that artists aren’t often the richest of people, sometimes they worked with what they could get. As Joris Dik of the Delft University of Technology, who has also researched the chemistry of paintings by Van Gogh and Rembrandt, notes in the Chemistry World article:

“Many of these artworks are really chemically dynamic, they’re not as static as people have thought.”

No actual restoration will be done for the red in the Renoir, although the digital restoration is being shown alongside the original in the gallery, which is itself presented in 360 degrees to display its full dimensions with the small bit of intact red as well. These behind-the-scenes conservation revelations are rarely brought to such detail in the galleries of a museum, but hopefully it won’t be the last to show how much our perception of a piece can be impacted as much by its degradation as its preservation.